AB-INITIO POTENTIAL-ENERGY SURFACE FOR THE COLLISIONAL SYSTEM H-+H-2 AND PROPERTIES OF ITS VAN-DER-WAALS COMPLEX

An accurate ab initio potential energy surface for the reaction H-2+H- has been determined by MR-CI and CEPA(2) calculations for 403 nuclear configurations. It is cast in analytical form by a fit with 23 parameters and a root-mean-square error of 0.27 mE(h) (0.07 mE(h) for all points below 50 mE(h)). The position and the height of the barrier have been found as r(b) = 1.997 bohr and 16.80+/-0.2 mE(h) (10.55+/-0.15 kcal/mol), respectively. Taking the change in zero-point energy into account, a threshold energy for the exchange reaction D2 + H- --> HD+D- of 0.49 eV is obtained, in good agreement with the experimental value of 0.42+/-0.12 eV. A linear van der Waals minimum has been found at r = 1.416 bohr and R = 6.183 bohr with a well depth of 1.75 mE(h) (1.10 kcal/mol). An accurate treatment of the zero-point motion leads to the new result that the H-3- complex is stable thermodynamically. The potential supports four bound vibrational levels. The electron detachment seam has been determined and its lowest point is found for R = 2.87 bohr, r = 1.42 bohr and gamma = 90-degrees with an energy of 1.2 eV, which again agrees comfortably with the experimental threshold of 1.2+/-0.1 eV for H-2 + D-.